Problem Set 2 [Word]

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Math 1111 Fall 2015 – Problem Set 2
1. Decipher the following message and identify the method used to construct the cipher alphabet.
You should assume it was enciphered using a monoalphabetic substitution cipher.
YGUWU HXAWU OAWKA LMENM QXUSA WAMMU MLUYC UUPYG USWQL MUOXQ RYGUS
GEXHN HXYAP TYGQX UQRYG UNWES YQIWA SGUWY GUXEX YUOQP CGHNG AOUXX AIUHX
UPNHS GUWUT NQWWU XSQPT XYQYG UMACX QRYGU ZPHBU WXUYG UHPYU WNUSY
UTOUX XAIUX YQYGU UBHTU PNUAB AHMAL MUYGU KUEXR QWATA EQWAO UXXAI
UYQHO SQWYA PYNQP XYAPY XCGHN GGABU YQLUT UYUWO HPUTY GUNQW WUXSQ
PTUPN UHXBU WENMQ XULZY YGUXZ LJUNY OAYYU WQRNW ESYQI WASGE HXBUW
EUAXH METUA MYCHY GLETH XNWUY UOANG HPUWE SGEXH NXPQY XQUAX HME
2. In a brief paragraph or two, describe the strategies you used to decipher the message above. How
did you use frequency analysis to start figuring out the cipher alphabet? What patterns in the
cipher alphabet did you notice that led you to determine the particular substitution cipher used?
What other steps did you take as part of your cryptanalysis?
3. How many possible transpositions of the plain text “SHERLOCK HOLMES” are there? How many are
there if you keep the words separate, that is, if you don’t allow the letters in “SHERLOCK” to
transpose with those in “HOLMES”?
4.
a. Suppose the 10 letters A, B, C, K, L, M, N, U, V, and W are each written on a tile and placed
in a bag. (You can imagine the game Scrabble, if that helps.) If you reach into the bag and
draw four tiles at random (without replacement), then lay them down in front of you in a
random sequence, what is the probability that the tiles spell the word LUCK?
b. Suppose the 17 letters G, E, T, T, Y, S, B, U, R, G, A, D, D, R, E, S, and S are each written on a
tile and placed in a bag. (Again, think Scrabble.) If you reach into the bag and draw five tiles
at random (without replacement), then lay them down in front of you in a random
sequence, what is the probability that the tiles spell the word TESSA?
5. Which of the following pairs of numbers are relatively prime? Justify your answers.
a. 45 and 54
b. 64 and 81
c. 105 and 122
d. 1155 and 1729
6. The least common multiple of two integers a and b, denoted LCM(a, b), is the smallest positive
integer that is divisible by both a and b.
a. Find LCM(15, 20).
b. Find LCM(10, 32).
c. Find LCM(37, 47).
d. Suppose that the positive integers m and n are relatively prime. Find a formula for LCM(m,
n) in terms of m and n. (You don’t need to prove your formula—that’s a much harder task
than making a good conjecture here.)
7. Consider the substitution cipher y = (7x + 6) MOD 26, where x is an integer between 0 and 25
representing a letter in the plaintext and y is an integer between 0 and 25 representing the
corresponding letter in the ciphertext. (Here, 0 = A, 1 = B, 2 = C, …, 25 = Z.) Decipher the ciphertext
“UAEUGJUDIV” using this cipher.
8. Consider the set of integers {0, 1, 2, 3}. Define the operation ◊ on this set as follows: If a and b are
integers in the set {0, 1, 2, 3}, then a ◊ b = (a + b) MOD 4. For instance, 2 ◊ 3 = (2 + 3) MOD 4 = 5
MOD 4 = 1. Note that 0 is the identity for this operation, because a ◊ 0 = a, no matter what a is.
For each of the integers in the set {0, 1, 2, 3}, find its inverse. That is, find the number from the set
that, when “added” to the target integer via the operation ◊, gives the identity 0.
9. The same plaintext message was enciphered using four different cipher techniques. A frequency
analysis of each of the four resulting ciphertexts was conducted and the results graphed below.
Match each frequency analysis with the cipher technique that produced it. In a sentence or two,
justify each of your answers.
a. Shift cipher +6 (the one that replaces “a” with “G”)
b. Shift cipher -10 (the one that replaces “a” with “Q”)
c. Transposition cipher
d. Vigenère cipher
Frequency Analysis #1:
Frequency Analysis #2:
Frequency Analysis #3:
Frequency Analysis #4: